Dominant periods and memory effect of the 2021 earthquake swarm in Hualien, Taiwan

Abstract

An earthquake sequence that occurred in the Hualien area during April 7 to August 30, 2021 has been recognized as two swarms by the Central Weather Bureau. Its earthquakes with ML ≤ 6.2 (ML = local magnitude) and focal depths ≤ 25 km were located in an area from 23°46’ N to 24°04’ N and from 121°25’ E to 121°42’ E. The Morlet wavelet technique is applied to analyze the dominant periods of temporal variations in numbers of daily events for the earthquake sequence in two magnitude ranges, i.e., ML ≥ 3 and ML ≥ 4. Results show that the dominant periods are 30.8 and 38.0 days when ML ≥ 3; while the dominant period does not exist when ML ≥ 4. The fluctuation analysis technique in the natural time domain is used to study the memory effect in the swarm for two magnitude ranges, i.e., ML ≥ 3 and ML ≥ 4. Calculated results show that the memory effect is stronger for the time sequence of magnitudes than for that of inter-event times and higher for ML ≥ 3 earthquakes than for ML ≥ 4 events. Consequently, only the short-term corrected memory effect was operative in the earthquake sequence of the Hualien swarms.